The present invention relates to electrochemical devices such as batteries, fuel cells, capacitors and sensors which employ electrically conductive ceramic materials, fibers, powder, chips and substrates therein to improve the performance of the electrochemical device.
There are numerous applications which involve the transfer of electrical current in environments which are highly corrosive or otherwise degrading to metallic conductors. Most notably are electrochemical devices operating under highly corrosive conditions and high temperatures. Examples of such applications are the use of electrodes for the chlor-alkali cell to make chlorine gas, electrodes for metal recovery, electrodes in hydrogen/oxygen fuel cells, electrodes for producing ozone, electrolysis of water and electrodes in high temperature solid oxide fuel cells. Most of these applications involve the contact of an electrode with an electrolyte under conditions which render the electrode ineffective during prolonged use. The loss of effectiveness can be gradual, such loss being manifested by reduced current-carrying capacity of the electrode. Exemplary types of conditions which render electrodes ineffective as they are used in current-carrying applications are described below.
One such condition involves chemical attack of the electrode by corrosive gas which is evolved from the electrolyte as it is decomposed during use. For example, the evolution of chlorine gas, a highly corrosive material, from an aqueous chloride-containing electrolyte such as, in the chlor-alkali cell is exemplary.
Another type of condition involves passivation of the electrode as it combines with the anions from the electrolyte to form an insoluble layer on its surface. This passivation condition occurs when the product from the electrochemical reaction can not diffuse from the electrode surface and this produces a blocking of the electrochemical sites and/or pores. The end result is a diminishing of the electrode current carrying capacity. An example of this passivation is the lead dioxide electrode in an aqueous sulfuric acid solution.
Another type of condition which renders electrodes ineffective involves the dissolution of the electrode by the electrolyte. The use of a zinc electrode in an aqueous potassium hydroxide solution is exemplary.
Various types of batteries such as secondary(rechargeable) batteries: lead-acid(Pb/PbO2), NaS, Ni/Cd, NiMH(metal hydride), Ni/Zn, Zn/AgO, Zn/MnO2, Zn/Br2; and primary(non-rechargeable) batteries: Zn/MnO2, AgCl/Mg, Zn/HgO, Al/Air(O2), Zn/Air(O2), Li/SO2, Li/Ag2CrO4 and Li/MnO2 exist.
Although a variety of batteries are available, the lead-acid battery remains favored for uses such as starting internal combustion engines, electric vehicle motive power, as well as portable and emergency power for industrial and military applications.
Lead-acid batteries include a cathode comprising a lead alloy grid (active material support structure and electrical network structure contact with the battery terminals) having PbO2 active material thereon; and an anode comprising sponge lead on a grid. The active material on a grid is called the plate and electrically, the anode (Pb) plate is negative and the cathode (PbO2) plate is positive. A separator, either glass fibers or porous plastic, is used to separate the cathode and anode from direct contact when the plates are in sulfuric acid electrolyte. For the lead-acid battery, the rated capacity (ampere-hours) depends on the total amount of electrochemically active material in the battery plates, the concentration and amount of sulfuric acid electrolyte, the discharge rate and the percent utilization (conversion of active material into ampere-hours) for the active materials (the cathode or PbO2 usually being the limiting factor).
During discharge of a lead-acid battery, the lead and lead dioxide active materials are converted to lead sulfate. The lead sulfate can form an undesirable, insulating layer or passivation around the cathode active material particles which reduces the active material utilization during discharge. This passivating layer can be the result of improper battery charging, low temperature operation, and/or excessive (high current) discharge rates. In order to increase the cathode active material utilization, which is desirable for battery performance, means to increase the cathode active material porosity which increases the amount of active material contact with the sulfuric acid and/or active material conductivity which minimizes resistance and electrical isolation of the active material particles are useful. However, raising the cathode active material porosity tends to increase the tendency for a loosening and possible loss of active material from the plate as well as electrical isolation of the active material from the grid structure. Wrapping the cathode plate with a glass mat holds the loosened active material tightly to the plate and minimizes the tendency for active material sediment (electrochemically lost cathode material) in the bottom of the battery container. The addition of conductive materials (carbon, petroleum coke, graphite) to increase the conductivity of the cathode active material is well-known, but these materials are degraded rapidly from the oxygen generated at the cathode during charging.
Since the lead-acid battery anode is very conductive, the additives for the sponge lead active material have concentrated on improving low temperature battery performance and cycle life. The fundamental additive to the anode is the expander which is comprised of lampblack, barium sulfate and lignosulfonic acid mixed with the lead oxide (PbO) carrier agent. The expander addition to the sponge lead inhibits densification or decrease in the sponge lead porosity. If the anode active material becomes too dense, it is unable to operate at low temperatures and can no longer sustain practical current discharges.
In the manufacture of lead-acid batteries, cathode electrodes are usually prepared from lead alloy grids which are filled with an active paste that contains sulfated lead oxide. This sulfated lead oxide is then later converted or formed into sponge lead for the anode and lead dioxide for the cathode. In an alternative construction, known as tubular cathode plates, the cathode active material is a sulfated lead oxide powder that is poured into a non-conductive tube (braided or woven glass or polyethylene) containing a protruding lead alloy rod or spine. Several of these tubes make up the grid structure and electrical connections are made to the terminals by the protruding lead alloy rods. The tubular cathodes and the usual plate anodes are then assembled into elements and these are then placed in a battery container. The cells are filled with electrolyte and the battery is subjected to the formation process. See details on lead-acid batteries, by Doe in Kirk-Othmer: Encyclopedia of Chemical Technology, Volume 3 (1978), page 640-663.
During lead-acid battery formation, active material particles in contact with the grid are formed first and particles further away from the grid are formed later. This tends to reduce the efficiency of formation. An apparent solution to this problem is addition of a conductive material to the active material paste. The additive should be electrochemically stable in the lead-acid system both with respect to oxidation and reduction at the potentials experienced during charge and discharge of the cell, as well as to chemical attack by the sulfuric acid solution. The use of barium metaplumbate and other ceramic perovskite powder and plating additives to the lead-acid battery anode and cathode are reported to enhance the formation of lead-acid batteries. See U.S. Pat. No. 5,045,170 by Bullock and Kao. However, these additives are limited to the lead-acid battery system and require up to a 50 weight percent addition to be effective.
For other battery systems, the cathode materials such as, MoO3, V2O5, Ag2CrO4 and (CFx)n that are used in primary lithium batteries are typically mixed with carbon, metal or graphite powder to improve the overall cathode electrical conductivity and therefore, the utilization of the cathode material. Depending on the battery design, the current collector is either the cathode material itself or a nickel screen pressed into the cathode material. The current collector for the anode (lithium) is a nickel screen pressed into the lithium metal. The separator between the lithium battery cathode and anode is typically a non-woven polypropylene, Teflon or polyvinyl chloride membrane. The electrolyte for the lithium battery is an organic solvent such as propylene carbonate, dimethyl sulphoxide, dimethylformamide, tetrahydrofuran to which some inorganic salt such as, LIClO4, LiCl, LiBr, LiAsF6 has been added to improve the solution ionic conductivity. Hughes, Hampson and Karunathilaka (J. Power Sources, 12 (1984), pages 83-144)) discuss the enhancement techniques used for improving the cathode electrical conductivity for lithium anode cells. While the addition of the materials to improve the cathode conductivity and utilization are feasible, the amount of additive material required means that much less electrochemical active cathode material that will be available, and in some lithium battery designs, because of volume limitations, that can be critical.
Other battery systems requiring that the cathode have improved conductivity and thereby, improved cathode (NiOOH/Ni(OH)2) active material utilization are secondary nickel batteries such as, Ni/Cd, Ni/Zn and Ni/MH (metal hydride). The electrolyte for the nickel battery system is usually potassium hydroxide solution and the separator between the anode and cathode is non-woven polypropylene. To enhance the cathode conductivity, graphite is added but this material is not long lasting as it is gradually oxidized to carbon dioxide. In addition to the degradation of the graphite, there is a gradual build-up of carbonate ions which reduces the conductivity of the electrolyte. See discussion on nickel batteries in xe2x80x9cMaintenance-Free Batteriesxe2x80x9d by Berndt.
A sodium-sulfur battery comprises molten sulfur or molten sodium polysulfide as a cathode, molten sodium as an anode, and a non-porous solid electrolyte made of beta alumina that permits only sodium ions to pass. The sulfur or sodium polysulfide in the cathode has an inferior electrical conductivity in itself. The art has attempted to address this problem by adding conductive fibers such as metal fiber or carbon fiber to the molten sulfur or molten sodium polysulfide. For general information, see U.S. Pat. Nos. 3,932,195 and 4,649,022. These types of fibers however, are prone to corrosion in the electrochemical environment of a sodium-sulfur battery. A need therefore continues for sodium-sulfur batteries which employ chemically stable conductive ceramic materials therein.
Another type of electrical energy generating device, as is known in the art, is the fuel cell such as acid fuel cells, molten carbonate fuel cells, solid polymer electrolyte fuel cells and solid oxide fuel cells. A fuel cell is an apparatus for continually producing electric current by electrochemical reaction of a fuel with an oxidizing agent. More specifically, a fuel cell is a galvanic energy conversion device that chemically converts a fuel such as hydrogen or a hydrocarbon and an oxidant that catalytically react at electrodes to produce a DC electrical output. In one type of fuel cell, the cathode material defines passageways for the oxidant and the anode material defines passageways for fuel. An electrolyte separates the cathode material from the anode material. The fuel and oxidant, typically as gases, are continuously passed through the cell passageways for reaction. The essential difference between a fuel cell and a battery is that there is a continuous supply of fuel and oxidant from outside the fuel cell. Fuel cells produce voltage outputs that are less than ideal and decrease with increasing load (current density). Such decreased output is in part due to the ohmic losses within the fuel cell, including electronic impedances through the electrodes, contacts and current collectors. A need therefore exists for fuel cells which have reduced ohmic losses. The graphite current collectors used in phosphoric acid and solid polymer electrolyte fuel cells, to the cathode metal oxides such as, praseodymium oxide, indium oxide used in solid oxide fuel cells and to the nickel oxide cathode used in molten carbonate fuel cells are examples of a need for conductive additives. See generally, xe2x80x9cHandbook of Batteries and Fuel Cellsxe2x80x9d, Edited by Linden.
Multilayer surface mount ceramic chip capacitors which store electrical energy are used extensively by the electronics industry on circuit boards. A typical multilayer surface mount chip capacitor is comprised of alternating multilayers of dielectric (ceramics such as BaTiO3) electrodes (metals such as Pd or Pdxe2x80x94Ag). The end caps or terminations of the capacitor are typically a metallic (Ag/Pd) in combination with a conductive glass. This termination is the means of contact to the internal electrodes of the multilayer ceramic capacitor. The development of other electrodes such as nickel and copper to reduce costs and the use of low cost conductive additives to the glass are actively being sought. See generally, Sheppard (American Ceramic Society Bulletin, Vol. 72, pages 45-57, 1993) and Selcuker and Johnson (American Ceramic Society Bulletin, 72, pages 88-93, 1993).
An ultra-capacitor, sometimes referred to as a super capacitor, is a hybrid encompassing performance elements of both capacitors and batteries. Various types of ultracapacitors are shown in xe2x80x9cUltracapacitors, Friendly Competitors and Helpmates for Batteries,xe2x80x9d A. F. Burke, Idaho National Engineer Laboratory, February 1994. A problem associated with an ultracapacitor is high cost of manufacture.
Sensors, as are known in the art, generate an electrical potential in response to a stimulus. For example, gas sensors such as oxygen sensors generate an electrical potential due to interaction of oxygen with material of the sensor. An example of an oxygen sensor is that described by Takami (Ceramic Bulletin, 67, pages 1956-1960, 1988). In this design, the sensor material, titania (TiO2), is coated on an alumina (Al2O3) substrate with individual lead connections for the substrate and the titania components. The development of higher electrical conductive titania to improve the oxygen sensor response is an on-going process. Another sensor, humidity, is based upon the electrical conductivity of MgCr2O4xe2x80x94TiO2 porous ceramics is discussed by Nitta et al. (J. American Ceramic Society, 63, pages 295-300, 1980). For humidity sensing, leads are placed on both sides of the porous a ceramic plaque and the sensor is then placed in the air-moisture stream for resistivity (inverse of electrical conductivity) measurements. The relative humidity value is then related to the measured resistivity value. With this design, the porous ceramic resistivity value, as low as possible, is critical because of the need for a rapid measurement response time (seconds) that can be related to an accurate relative humidity value.
Another type of electrical device, as is known in the art, is a bipolar battery. Such a battery typically comprises an electrode pair constructed such that cathode and anode active materials are disposed on opposite sides of an electrically conductive plate, that is, a bipolar plate. The cells that have this electrode pair are configured such that the cell-to-cell discharge path is comparatively shorter and dispersed over a large cross-sectional area, thus providing lower ohmic resistance and improved power capabilities compared to unipolar batteries such as automobile batteries. The bipolar electrodes are stacked into a multicell battery such that the electrolyte and separators lie between adjacent bipolar plates. The Lead-acid batteries are attractive candidates for bipolar construction because of the high power capabilities, known chemistry, excellent thermal characteristics, safe operation and widespread use. However, such lead-acid batteries with bipolar construction often fail due to the corrosion of the electrically conductive plate when in contact with the active material. A need therefore exists for bipolar batteries which have improved corrosion resistance, low resistivity and reduced weight. For general information on bipolar batteries, see Bullock (J. Electrochemical Society, 142, pages 1726-1731, 1995 and U.S. Pat. No. 5,045,170) and U.S. Pat. No. 4,353,969.
Although the devices of the prior art are capable of generating and storing electrical energy, and acting as oxygen and relative humidity sensors, there is a need for improved materials of construction for reasons of diminished corrosion, higher capacity and/or higher electrical conductivity which overcome the disadvantages of the prior art.
In addition to the previously mentioned materials used in the above applications, there are several U.S. Patents which delineate the electrochemical use of electrically conductive ceramics such as the sub-oxides of titanium which are formed from the reduction of titanium dioxide in hydrogen or carbon monoxide reducing gases at high temperatures (1000xc2x0 C. or greater). For example, U.S. Pat. No. 5,126,218 discusses the use of TiOx (where x=1.55 to 1.95) as a support structure (grids, walls, conductive-pin separators), as a conductive paint on battery electrodes and as powder in a plate for the lead-acid battery. A similar discussion occurs in U.S. Pat. No. 4,422,917 which teaches that an electrochemical cell electrode is best made from bulk material where the TiO. has its x vary from 1.67 to 1.85, 1.7 to 1.8, 1.67 to 1.8, and 1.67 to 1.9.
The said mentioned electrode materials are suitable for electrocatalytically active surfaces when it includes material from the platinum group metals, platinum group metal alloys, platinum group metal oxides, lead and lead dioxide. The electrodes are also suitable for metal plating, electrowinning, cathodic protection, bipolar electrodes for chlorine cells, tile construction, and electrochemical synthesis of inorganic and organic compounds.
Oxides of titanium are discussed in U.S. Pat. No. 5,173,215 which teaches that the ideal shapes for the Magneli phases (TinO2nxe2x88x921 where n is 4 or greater) are particles that have a diameter of about one micron (1 micron (denoted xcexc) is 10xe2x88x926 meter (denoted m)) or more and a surface area of 0.2 m2/g or less.
The U.S. Pat. No. 5,281,496 delineates the use of the Magneli phase compounds) in powder form for use in electrochemical cells. The use of powder is intended for the electrode structure only.
U.S. Pat. No. 4,931,213 discusses a powder containing the conductive Magneli phase sub-oxides of titanium and a metal such as copper, nickel, platinum, chromium, tantalum, zinc, magnesium, ruthenium, iridium, niobium or vanadium or a mixture of two of more of these metals.
The invention is directed to solving the problems of the prior art by improving electrochemical devices such as batteries, fuel cells, capacitors, sensors and other electrochemical devices as follows: (1) In batteries for example, there will be an improved discharge rate, increased electrochemically active material utilization, improved charging efficiency, reduced electrical energy during the formation of electrochemically active materials and decreased electrical resistance of the electrochemically active material matrix; (2) In fuel cells, for example, there will be decreased electrical resistance of the current collectors and cathode materials as well as increased electrical chemical efficiency of the reactants; (3) In capacitors, for example, there will be development of less expensive electrodes and conductive glass; (4) In sensors, there will be the development of lower resistive titanium dioxide for oxygen sensors and lower resistive binary compounds containing titanium dioxide for relative humidity sensors; and (5) In other electrochemical devices, for example, there will be the development of more corrosion resistant and current efficient electrodes for electrolysis, electrosynthesis.
As used herein, conductive ceramic materials include conductive ceramic compositions such as solids, plaques, sheets (solid and porous), fibers, powders, chips and substrates (grids, electrodes, current collectors, separators, foam, honeycomb, complex shapes for use in components such as grids made by known methods such as weaving, knitting, braiding, felting, forming into paper-like materials, extrusion, tape casting or slip casting) made from conductive ceramic compositions having metal containing additives and metallic coatings thereon, or made from non-conductive ceramic compositions having metal containing additives and metallic coatings dispersed thereon.
The electrically conductive ceramic materials for use in the invention, when in the form of fibers, powders, chips or substrate, are inert, light weight, have high surface area per unit weight, have suitable electrical conductivity, as well as high corrosion resistance. Typically, the electrically conductive ceramic fibers, powders, chips or substrate herein have an electrical conductivity of 0.1 (ohm-cm)xe2x88x921 or more.
Electrically conductive ceramic fibers, powder, chips or substrate useful in the invention include an electrically conductive or non-electrically conductive ceramic matrix, preferably with a metal containing additive and/or metallic coating. Ceramic matrix materials which may be employed include, the oxides of the metals titanium, and vanadium and the oxides of zirconium, and aluminum. The reduced oxides of titanium and vanadium have a certain amount of intrinsic electrical conductivity and the oxides of zirconium and aluminum are intrinsically insulators. All mentioned ceramic oxides have different chemical and physical attributes and these materials cover a wide range of applicability. Either ceramic metal oxide can have the electrical conductivity increased by the addition or plating or coating or deposition of singly or a mixture thereof of metallic d-block transition elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, La, Hf, Ta, W, Re, Os, Ir, Pt, Au), Lanthanides (Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, Lu) and/or by the addition or plating or coating or deposition singly or a mixture thereof of selected main-group elements (B, Al, Ga, In, Tl, Si, Ge, Sn, Pb, As, Sb, Bi, Se, Te) and/or the oxides, halides, carbides, nitrides and borides of the aforementioned elements. Chemical reduction processes for the selected mixtures reduce the ceramic to its final electrically conductivity form. Similarly, chemical oxidation processes may be used to form superstoichiometric titanium oxide in which the atomic oxygen to titanium ratio is slightly above 2.
The electrically conductive ceramic materials, fibers, powders, and chips mentioned in this invention can be used to enhance the electrical conductivity and thereby, the utilization of the electrochemically active materials in cathode for the following primary and secondary battery systems: lithium batteries, zinc air batteries, aluminum air batteries, alkaline batteries, Leclanche batteries, nickel batteries, lead-acid batteries, and sodium-sulfur. The electrically conductive ceramic substrate as mentioned in this invention would be suitable for fuel cell electrodes and current collectors and bipolar plate batteries. In addition, the electrically conductive ceramic materials, fibers, powders, chips and substrate according to this invention would be suitable for oxygen and humidity sensors as well as multilayer chip capacitors and ultracapacitors. The electrode made from this invention can also be useful as an anode or cathode, whichever is applicable, in electrochemical devices including batteries and in an electrolytic cell generating ozone, chlorine gas, or sodium, recovering metals from wastewater and purification of metals by electrolysis.
The electrically conductive ceramic materials, fibers, powders, chips and/or substrates therein may impart superior battery discharge and charging performance, battery cycle life, battery charge retention, battery weight reduction, deep battery discharge recovery, as well as battery structure vibration and shock resistance. Batteries such as lead-acid batteries which employ electrically conductive ceramic materials, fiber, powder, chips and/or substrates therein advantageously may require reduced electrical energy during formation. Fuel cells utilizing electrically conductive ceramic materials, fibers, powder and/or substrate for the current collector and the electrodes may have longer operating life because of superior corrosion resistance and enhanced performance because of superior electrical conductivity. The use of electrically conductive ceramic materials, fibers, powder, chips and substrates from this invention may impart low cost manufacturing, superior electrical resistivity performance in oxygen and humidity sensors, multilayer chip capacitors, and ultracapacitors.
Other advantages of the present invention will become apparent as a fuller understanding of the invention is gained from the detailed description to follow.